Probably you could achieve that using "RunScriptCode" input to set the tank health.
Maybe would need a trigger_once entity as well to set it.
This is how is done in back4glub map just to give you some hint:
PHP Code:
{
"targetname" "trigger_tank"
"StartDisabled" "0"
"spawnflags" "3"
"origin" "999 999 999"
"filtername" "filter_tank"
"classname" "trigger_once"
"OnStartTouch" "!activator,RunScriptCode,self.SetHealth(self.GetHealth() *1.5),0,-1"
"OnStartTouch" "!activator,SetColor,255 0 0,0,-1"
"OnStartTouch" "!self,Kill,,0,-1"
}
{
"origin" "999 999 999"
"targetname" "filter_tank"
"Negated" "Allow entities that match criteria"
"filterinfectedclass" "8"
"classname" "filter_activator_infected_class"
}
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